The source says, "Kristen is terrified that Rob is going to leave her for someone else. She has always been insecure about her boobs, and she is totally convinced Rob would find it a massive turn-on if she had even a subtle size increase."

(Um, you've gotta be kidding me.)

Before I go any further, let me just reiterate that I'll be absolutely shocked if there is one hint of truth to this story.

And I really, really hope there isn't -- because Kristen's in for a rude awakening if she truly thinks increasing her bust size will make Rob less likely to leave her.

First of all, he just doesn't strike me as a boob dude. Considering how loyal he's been to her over the past few years, it's obvious that he finds her other traits (like intelligence, personality, and maturity) much more important than her breast size.

Second, while he may be initially very turned on if she happened to go out and splurge on a pair of new "girls," the novelty will wear off eventually. It's not like a breast enhancement will seal the deal as far as them staying together forever goes.

And lastly, if Kristen (or any woman for that matter) feels the need to resort to plastic surgery in an effort to keep her man, then odds are good that the relationship isn't worth hanging on to in the first place.

Kristen looks great just the way she is, and I don't think I'm going out on a limb by saying I'm pretty sure Rob agrees.